Production of acetic acid by methanol carbonylation is known. See U.S. Pat. No. 5,817,869. In the current acetic acid production process, a reaction mixture is withdrawn from the reactor and is separated by a flash tank into a liquid fraction comprising the catalyst and the catalyst stabilizer and a vapor fraction comprising the acetic acid product, the reactants, water, methyl iodide, and impurities generated during the carbonylation reaction. The liquid fraction is then recycled to the carbonylation reactor. The vapor fraction is then passed to a so-called “light-ends distillation.” The light-ends distillation separates acetic acid from other components.
The light ends distillation separates an overhead fraction comprising methyl iodide, water, methanol, and methyl acetate from an acetic acid stream comprising acetic acid, a small amount of water, and heavy impurities. The acetic acid stream is passed to a drying column to remove water and then be subjected to a so called “heavy-ends distillation” to remove the heavy impurities such as propionic acid.
One problem associated with the current process is that the catalyst, usually expensive rhodium or iridium metals, cannot be completely recovered because it is entrained in the vapor phase or affixed on the wall of the flash tank.
Another problem associated with the current process is that the catalysts are often deactivated during the flashing. Rhodium and iridium catalysts need a high concentration of water to stabilize. During flashing, a significant amount of water goes with the vapor phase, and thus the rhodium and iridium catalysts become unstable due to lack of sufficient water.
Further, equipment for acetic acid production requires high corrosion resistance. The flash tank is often constructed of corrosion resistant metals such as Hastelloy B-2 and Zirconium 702. These metals are very expensive.
A new process for producing acetic acid is needed. Ideally, the process would eliminate the flash tank.